Metal matrix composites consist of a wide array of microstructures. These composites either consist of a metallic or an intermetallic compound as the contiguous matrix. Usually the reinforcing material consists of a ceramic; however, a refractory metal is sometimes used. Reinforcing material may be in the form of particulates, wiskers, or discrete fibers either continuous, long or short The reinforcing material can be consolidated as porous preforms in certain cases to facilitate the formation of the composite.
Observations of cast microstructures of metal matrix particulate composites (MMC), such as Al--Si--SiC, Al--Si--Al.sub.2 O.sub.3 and Al--Si--Gr indicate that during solidification, the particulates or fibers tend to segregate at the boundaries of proeutectic .alpha. Al dendrites instead of being engulfed within the body of growing dentrites. This phenomenon produces a nonuniform distribution of reinforcements in the matrix, and prevents the realization of the full potential of properties of cast metal matrix composites.
In an effort to improve the distribution of reinforcements and to understand the segregation phenomenon, significant amount of research has been conducted by several investigators. In general, most of the experimental results and suggested theories fall into two classes, namely,
(a) Unfavorable thermodynamic conditions for the nucleation of aluminum phase on dispersoids and their engulfment by the growing primary aluminum phase. PA1 (b) Pushing of particles by the solidifying .alpha. aluminum dendrites into the last freezing interdendritic regions. PA1 1. Graphite tube, commercial quality, 3 mm ID 6 mm OD PA1 2. Graphite (pencil) leads 0.6, 0.7 and 0.9 mm in diameter PA1 3. Graphite fiber tows coated with nickel PA1 (a) As squeeze cast without remelting. PA1 (b) Squeeze cast, remelted and solidified while the ends of fiber extending outside the composite were water cooled.
The absence of nucleation of aluminum on the surface of the reinforcement (heterogeneous nucleation) may result from differences in surface and interfacial energies of the particles and the solidifying phase, as well as from possible differences in temperatures between the solidifying liquid in the immediate vicinity of a particle, and that of the particle surface itself. The differences in temperature between these solidifying liquid and the particulates may be small, and yet finite. These temperature differences between the particles and the liquid around the particles can result from the differences of thermal properties of aluminum and the ceramic reinforcements over the temperature range during which solidification takes place. Variation in thermal conductivity of some ceramic reinforcements as a function of temperature is shown in FIG. 1. Typical thermal properties of the Al--Si alloy and selected reinforcements are given in Table 1 indicating significant differences in thermal diffusivity and heat diffusivity of the matrix alloys and the reinforcements.
In an effort to illustrate the possible role of differences in temperature between the particles and the melt on solidification microstructures in cast composites, observations of solidification microstructures around larger diameter graphite tubes were made when the surface temperatures of the tubes were controlled by internal means. As a first step Al--Si alloy was solidified around tubes, rods and of graphite fibers. The influence of changes in thermal conditions of the graphite tubes, rods and fibers surfaces on the solidification microstructure of Al--Si alloys has been examined. Mechanical properties of fiber reinforced tensile samples were measured for samples where the fibers were cooled during solidification, to illustrate the influence of cooling of fibers on mechanical properties.